The present invention relates to a method and an apparatus for reviewing a defect occurring on a sample such as a semiconductor wafer in a process for manufacturing a semiconductor, and more particularly to a method and an apparatus for reviewing a defect detected by an external optical inspection apparatus by using a scanning electron microscope (SEM) in detail.
When a defect such as a foreign material is existed on a semiconductor substrate (wafer) in a process for manufacturing a semiconductor, a circuit failure may occur. The circuit failure includes insufficient insulation and a short circuit occurring in a wiring connection. In accordance with a semiconductor device miniaturization, a minute defect may cause a capacitor to be insufficiently insulated or a gate oxide film to be broken. Defects may be introduced into a sample in various states due to various causes. For example, a defect may be introduced into a sample from a movable part of a carrier device, a human body, a chemical or a material, or may be a product generated by reaction with a process gas present in a process device. To avoid the problem, it is necessary to change process conditions or improve equipment for manufacturing semiconductors, by detecting a defect occurring on a semiconductor substrate in the manufacturing process and finding the source of the defect.
In a conventional method for detecting the source of a defect, an external optical inspection apparatus specifies the position of the defect, and a scanning electron microscope (SEM) or the like reviews the defect in detail and classify the defect. Then, information of the defect reviewed in detail and classified is compared with information present in a database, and the cause of the defect is estimated based on the comparison.
The external optical inspection apparatus is an optical foreign material inspection apparatus or an optical appearance inspection apparatus. The optical foreign material inspection apparatus is adapted to detect light scattered from the defect to specify the position of the defect. The optical appearance inspection apparatus is adapted to detect a bright field optical image of a semiconductor substrate by using lamp light or laser light as illumination light to compare the optical image with reference information thereby specifying the position of the defect on the semiconductor substrate.
Such reviewing methods are disclosed in JP-A-2001-133417 corresponding to U.S. Pat. No. 6,407,373 B1, JP-A-7-325041, JP-A-2003-7243, JP-A-5-41194, JP-A-2005-156537 corresponding to US 2005/0122508 A1 and JP-A-2007-71803 corresponding to US 2007/0057184 A1.
To detect a defect present on a semiconductor substrate by using an optical foreign material inspection apparatus with high inspection throughput, laser light is irradiated on the semiconductor substrate with a large spot to scan the substrate and detect the defect. This results in the fact that information on the position of the defect, which is obtained based on the position of the laser spot to scan the semiconductor circuit, includes a large error component.
When the defect is reviewed by a scanning electron microscope in detail with higher magnification (a smaller visual field) than that of an optical system of the external optical inspection apparatus based on rough position information of the defect with the error component, the reviewed defect may not fall within the visual field of the scanning electron microscope. To cause the defect (to be reviewed) to fall within the visual field of the scanning electron microscope, the defect is searched while the visual field of the scanning electron microscope is changed. This takes a long time since the visual field is small, resulting in a reduction in the throughput of the scanning electron microscope and an increase in the time for analyzing the defect.
JP-A-2001-133417 discloses an apparatus for reviewing a defect arranging both a scanning electron microscope and an optical microscope to solve the abovementioned problem. In the apparatus described in JP-A-2001-133417, the optical microscope redetects a defect and specifies the position of the defect, and the scanning electron microscope reviews the defect in detail. It is general that when a numerical aperture of an objective lens provided in the optical microscope is increased in order to detect a foreign material with high sensitivity, a focal depth of the optical microscope is reduced. This results in difficulty in the focusing operation of the optical microscope. In JP-A-2001-133417, it touches about a focusing operation of the optical microscope. JP-A-2001-133417, however, does not provide a sufficient description of the focusing operation of the optical microscope. JP-A-2005-156537 describes a focusing method. In the focusing method described in JP-A-2005-156537, a position in a Z-direction at which in a plurality of dark field images detected by moving a Z-stage step-by-step in predetermined displacements in the Z-direction, a luminance value of the detected dark field image is the maximum is deemed as a just-in-focus position. In the focusing method, however, when a numerical aperture of an objective lens provided in an optical microscope is increased, a focal depth of the optical microscope is reduced. This results in the fact that it is necessary to reduce the displacement of step-by-step for acquiring a dark field image. The throughput of the focusing operation is therefore reduced.
In addition, it is difficult to detect a foreign material on a metal film with high sensitivity by an optical microscope as a defect present on a semiconductor substrate, even when the numerical aperture of an optical system provided in the optical microscope is large.